This invention relates to a method of measuring circularity of an optical fiber.
Referring to FIG. 1, an optical fiber is composed of a core 12 of high refractive index material surrounded by a cladding 14 of low refractive index material. Ideally, the core and the cladding are substantially circular in cross-section and the core is centered within the cladding, as shown in FIG. 1. However, current techniques for manufacture of optical fibers do not ensure that the cladding is circular when viewed in cross-section and it may in fact be more or less elliptical, as shown in FIG. 2. An important measure of the quality of an optical fiber is the non-circularity of the cladding, which is defined as
      NC    ⁢    %    =                    A        -        B                              (                      B            +            A                    )                /        2              ×    100  where A is the maximum diameter of the cladding measured using a contact micrometer or similar technique and B is the diameter measured along an axis that is perpendicular to the diameter along which the maximum diameter is measured. It will be understood that in the case of an elliptical cladding, the ellipticity of the cladding may be defined as B/A. The value (B+A)/2 may be specified as the diameter of the cladding.
It is known to measure geometrical characteristics of an optical fiber by cleaving the fiber and carrying out measurements on the end face of the optical fiber employing imaging optics to project an image of the end face on a CCD sensor, which generates an electrical signal representative of intensity of illumination of the sensor as a function of position in the image plane by reference to a Cartesian coordinate system. This electrical signal can be digitized and the resulting data stored as an electronic representation of the optical image that is projected into the image plane by the imaging optics. A video signal for driving a monitor to display a visually perceptible image of the end face of the fiber can be generated by reading out and appropriately processing the electronic image data. See U.S. Pat. No. 6,421,118.
Using conventional image processing software, it is possible to analyze the electronic image data generated by the imaging optics and the CCD sensor and identify points that lie on the periphery of the electronic representation of the cladding, calculate the locations of these points in the electronic representation, and calculate the equation of an ellipse 16 (FIG. 2) that optimally fits the locations of the points. It is then possible to calculate the values of the non-circularity and diameter of the cladding.
If the imaging system (imaging optics and CCD sensor) were perfect, the calculated values of the fiber parameters would be equal to the actual values of the parameters. However, imperfections in the imaging optics may introduce errors such that the image of the cladding is distorted. The distortion may result in the image of a circular cladding being elliptical. Further, imperfections in the CCD sensor may introduce errors in measuring the location of a feature in the image plane. Consequently, unless one is able to sufficiently calibrate the imaging system, it is not possible to know a geometric parameter of the cladding simply by acquiring and analyzing an electronic image of the end face of the fiber.